drm/i916: Refactor WM register maximums

We will have another use for the maximum watermark values that the
registers can hold. Pull those out into separate functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>